Although the precise pathogenesis of AD is unknown, certain pathological features accompany the disease. These pathological features include the abnormal accumulation of extracellular amyloid, the formation of intraneuronal neurofibrillary tangles, synapse loss, and cellular degeneration. Cellular degeneration occurs in several neuronal populations in the central nervous system. Among the neuronal populations that degenerate in AD, loss of basal forebrain cholinergic neurons is particularly severe. Loss of cholinergic neurons in AD correlates best with severity of dementia, the density of amyloid plaques in the brain, and the amount of synapse. To date, the only FDA-approved therapies for Alzheimer's Disease focus on augmenting the function of degenerating cholinergic neurons.